The invention relates to a Stirling cooler, comprising a compressor for generating a time-varying pressure in a gaseous medium, a cooling element provided with at least a displacer and at least a regenerator and additionally comprising a connecting line between the compressor and the cooling element.
Stirling coolers of this type are well-known and are mostly used for generating extremely low temperatures, in the order of 80 K, for instance for cooling optical sensors incorporated in infrared cameras. The advantage of inserting a connecting line between compressor and cooling element is that it provides maximum flexibility in the design of the system that is to accommodate the Stirling cooler. This enables the compressor to be mounted at a distance of the object to be cooled. The compressor is usually quite sizeable in comparison with the cooling element which may include a so-called cold finger. The connecting line, the length of which may vary from a few centimeters to several decimeters, enables the cooling element to be mounted at a certain distance from and at a random position with respect to the compressor.
The connecting line also goes by the name of split tube. The split tube mostly has a diameter ranging from less than one to several millimeters. A cooling medium, for instance Helium, is alternately compressed and expanded at a high frequency of for instance 50 Hz. The consequent periodically fluctuating pressure in the system is transmitted via the split tube to the cooling element. A cooling element implemented as a cold finger usually comprises a cylindrical cavity containing a displacer, which may also serve as a regenerator. The split tube is usually connected to the cooling element at the base of the displacer. To ensure the cooler's operational effectiveness, the displacer motion shall be tuned to the pressure fluctuations. To this end, the displacer motion shall preferably be 90.degree. out of phase with the pressure. To achieve this, the displacer can be spring-mounted such that it will perform a reciprocating motion caused by the flow of cooling medium along the displacer, which yields a phase lag of approximately 90.degree. with respect to the pressure fluctuation. The fluctuating pressure and the displacer motion give rise to a difference in temperature between the top and base of the displacer, which phenomenon is known from thermodynamics. Because of this temperature difference, reference is also often made to the warm side and the cold side of the cooling element, representing its base and top respectively.
A drawback attached to these types of Stirling coolers provided with a split tube is that the warm side of the cooling element is additionally heated as a result of heat conveyed via the split tube from the compressor to the cooling element. The temperature increase may assume such proportions that the required cooling power is no longer realized thus resulting in a rise of temperature on the cold side of the cooling element.